Complementary Metal Oxide Semiconductor (CMOS) area image sensors and Metal Oxide Semiconductor (MOS) area image sensors (both hereinafter referred to as MOS sensor), as well as Charge Coupled Devices area image sensors (hereinafter referred to as CCD sensor), generate image signals based on an amount of charges created depending on the intensity of incident light coming to the sensors. As functional devices, these image sensors (solid-state imaging devices) are included in various kinds of imaging appliances such as digital still cameras, digital camcorders, IP cameras, and cell-phone cameras.
A conventional image sensor has pixel units (unit pixels) arranged in a two-dimensional array near the surface of a semiconductor substrate, the pixel units each including a photoelectric converting unit (photo diode) and a readout circuit unit. The opening area of the photoelectric converting unit reduces depending on the area of the reading circuit unit. Hence, the conventional image sensor is problematic in that the aperture ratio of the photoelectric converting unit drops with the decreasing size of the pixel unit.
In contrast, Patent Literature 1 cites a stacked image sensor including a readout circuit unit provided near the surface of the semiconductor substrate, and a photoelectric converting film made of a material which allows photoelectric conversion and stacked above the solid-state imaging device.
The photoelectric converting unit for a stacked image sensor in PTL 1 includes a photoelectric converting film provided between electrodes. The photoelectric converting film generates charges from light signals, and the generated charges are transferred to a readout circuit via the electrodes. FIG. 15 depicts the structure of a pixel unit in PTL 1. In the pixel unit of FIG. 15, the charges generated in a photoelectric converting film 101 are accumulated through a pixel electrode 102 in a charge accumulating unit 103 which is a p-n junction and formed on the substrate. The charge accumulating unit 103 is connected to the gate of an amplifying transistor 104. The pixel unit outputs a voltage variation due to a fluctuation in the amount of the accumulated charges. The charge accumulating unit 103 is further connected to the drain of a reset transistor 106 in order to reset the charges in the charge accumulating unit 103 after the charges are read out of the photoelectric converting film 101. When the reset transistor 106 turns on, the voltage of the charge accumulating unit 103 is set to an initial voltage.